1. Field of the Invention
The present invention relates to direct contact condensation and, more particularly, to an improved direct contact condenser apparatus for use in a geothermal power plant, and a method of condensing geothermal vapor utilizing same. The present invention also relates to a method for predicting the performance of an improved direct contact condenser.
2. Description of the Prior Art
Geothermal energy resources have generated considerable interest in recent years as an alternative to conventional hydrocarbon fuel resources. Fluids obtained from subterranean geothermal reservoirs can be processed in surface facilities to provide useful energy of various forms. Of particular interest is the generation of electricity by passing geothermal vapor through a steam turbine/generator.
The construction and operation of geothermal power plants would be simplified if the low-pressure effluent from the steam turbine were exhausted directly into the atmosphere. However, geothermal fluids typically comprise a variety of potential pollutants, including noncondensable gases such as ammonia, hydrogen sulfide, and methane. Because of these contaminants, particularly hydrogen sulfide, discharging a geothermal vapor exhaust into the atmosphere is usually prohibited for environmental reasons. Thus, the conventional approach is to exhaust the turbine effluent into a steam condenser to reduce the turbine back pressure and concentrate the noncondensable gases for further downstream treatment.
Many geothermal power plants utilize direct contact condensers, wherein the cooling liquid and vapor intermingle in a condensation chamber, to cool and condense the vapor exhausted from the turbine. Direct contact condensers are generally preferred over surface condensers, in which the vapor and cooling liquid are separated by the surface of the conduit through which the cooling liquid flows, because of the former's relative simplicity and low cost. However, to realize optimal heat transfer efficiency using direct contact condensers, the cooling liquid must be introduced into the condensation chamber at a high enough velocity to disperse the liquid into fine droplets, i.e., to form a rain, which increases the surface area for condensation. Unfortunately, this high velocity discharge reduces the contact time between the cooling liquid and the vapor, which in turn reduces the heat exchange efficiency. Consequently, conventional direct contact condensers require relatively large condensing chambers to compensate for this low heat transfer efficiency and to provide sufficient contact between the liquid and vapor to effect condensation.
One way to increase the condensation efficiency, and thus minimize the size of the direct contact condenser, is to inject the cooling liquid through a plurality of individual nozzles, which disperses the cooling liquid in the form of a film. Because a film provides greater surface area for condensation than normal liquid injection, the cooling liquid can be introduced into the chamber at perhaps a lower rate and a lower injection pressure, i.e., without generating a rain of fine droplets. Although these spray-chamber condensers offer somewhat improved condensation efficiency and more compact designs than previous generation condensers, they require substantial quantities of cooling liquid to obtain sufficient condensation. Therefore, because of the additional energy requirements and losses associated with pumping the excess cooling liquid to the condensation chamber, the practical efficiency of these condensers remains low.
Subsequent developments have focused on improving the efficiency of contact between the vapor and cooling liquid by modifying the liquid injection and/or dispersion mechanisms. U.S. Pat. No. 3,814,398 to Bow, for example, discloses a direct contact condenser having a plurality of spaced-apart deflector plates angularly disposed relative to the cooling liquid inlet. The deflector plates are positioned to break up the cooling liquid into liquid fragments, thus generating a film of coolant. The condenser includes multiple spray chambers, wherein each chamber has deflector plates and a liquid conduit. Obvious disadvantages of this design are its complexity and high cost due to the large numbers of partitions, deflector plates, and liquid conduits required to generate the film.
In addition to spray chambers, heat transfer between the cooling liquid and the vapor in direct contact condensers has been accomplished using baffle tray columns, cross-flow tray columns, and pipeline contractors (J. R. Fair, Chemical Engineering, 2:91-100 (1972); J. R. Fair, Chem. Eng'g Prog. Symp., 68(118):1-11(1972); and J. R. Fair, Petroleum and Chemical Engineer, 2:203-210 (1961)). Unfortunately, all of these designs yield generally low (60-70%) condensation efficiencies due to back-mixing. Moreover, many such condensers, particularly cross-flow tray condensers, involve a long, tortuous path for the vapor flow from the vapor inlet to the noncondensable gas outlet. To provide this long, tortuous vapor path, such devices require a large housing and a complex internal network. In addition to being difficult and costly to produce, these conventional designs generally suffer from high condenser back pressures as a result of the tortuous vapor path. Finally, most of these conventional designs, and baffle-column designs in particular, suffer from large gas-side pressure losses due to the generally high concentrations of uncondensed vapor in the exhausted noncondensable gas stream. Considerable gas-side pressure losses thus result from the additional energy requirements associated with pumping this residual vapor from the condensing chamber and reduce the useful power that can be extracted from the turbine.
Direct contact condensers have also been designed using packed columns as the liquid-vapor contact medium to improve the efficiency of contact between the vapor and cooling liquid. However, such packed columns are typically randomly distributed and thus create a complex vapor flow pattern. Because of this complex flow pattern, packed-column condensers suffer from some of the same drawbacks as the cross-flow tray condensers, namely, high condenser back pressures and large gas-side pressure losses.
Another significant concern regarding geothermal vapor processing relates to the presence of certain noncondensable gases, as discussed above. When this contaminated vapor is mixed with the cooling liquid in the condensation chamber, a portion of the noncondensable gases dissolves in the liquid. These noncondensable gases tend to diffuse between the condensate-cooling liquid mixture and the gas stream. The relative concentrations of contaminants in the liquid and gas streams depend upon the geometry of the condenser and fluid property (e.g., temperature and pressure) conditions within the condenser. In practice, these contaminants typically cause both the liquid and gas effluents from the condenser to be corrosive and/or toxic. Although various processes have been developed for pollution abatement at geothermal power plants, most such processes involve expensive chemical treatments and often do not provide acceptable abatement of emissions at a reasonable cost. Moreover, from both environmental and economic perspectives, it would be advantageous to segregate the more highly contaminated condensate mixture from the spent cooling liquid. It would be desirable to separate these two liquids so that the contaminated portion can be effectively treated, while the less contaminated cooling liquid is returned to the cooling tower and safely recycled. Unfortunately, none of the existing direct contact condensers provide a mechanism for effectively concentrating the contaminants in one fraction and separating this contaminated fraction from the relatively innocuous cooling liquid stream.
In addition to environmental concerns, the noncondensable gases present in geothermal vapor can accumulate in the condensation chamber, thus adversely affecting the efficiency of the turbine and/or condenser, and impairing overall plant performance. Unless removed, these gases will collect in the condenser, blanketing the condensing surfaces and reducing the surface area for condensation. These accumulated contaminants also increase the pressure within the condensation chamber, thus affecting the turbine back pressure. Moreover, hydrogen sulfide readily dissolves in the cooling liquid, where it oxidizes to form sulfurous acid and sulfuric acid, both of which are strongly corrosive to many metals. Thus, to maintain a suitable operating pressure within the condenser and to minimize corrosion and fouling of equipment, additional pumping or compression power must be expended to remove these gases.
Another problem commonly associated with existing condensers is the difficulty in achieving uniform distribution of cooling liquid across the condenser housing. To achieve optimum efficiency, it is important that the coolant be dispersed uniformly throughout the condensing chamber to facilitate mixing with the vapor and to maximize the available area for condensation. Moreover, it is well known that, in devices having cooling liquid injection in the upflow stage, vapor may condense mostly near the bottom, which is desirable, or may condense mostly on top, because of upsets. This switching between the two modes of operation is typically termed bang-bang instability. Thus, it is desirable to include an automatic and intermittent cooling liquid discharge operation in the upward flow stage, wherein additional cooling liquid is supplied during periods of operational instability and/or high vapor flow through the upflow stage. Finally, direct contact condensers suitable for use in a geothermal power plant must also be inexpensive, compact, and simple in design. Appropriate engineering methods to develop such designs must also be available.
A need therefore exists for an improved, high efficiency direct contact condenser for use in a geothermal power plant. This improved condenser should include a vapor-liquid contact medium to facilitate contact between the vapor and cooling liquid, a relatively short and straight vapor flow path to minimize the condenser back pressure and vapor pressure losses, and a separate hot well for effluents containing relatively high concentrations of noncondensable gases. This high efficiency condenser should also provide uniform distribution of cooling liquid, an automatic and intermittent liquid discharge system in the upflow stage, and be inexpensive, compact, easy to maintain, and simple in design. A need also exists for a method of condensing vapor from a geothermal power plant which eliminates or minimizes the efficiency and environmental concerns commonly associated with the direct contact condensation of geothermal vapor. Finally, a need exists for a method of predicting the performance of a direct contact vapor condenser. Until this invention, no such device or methods existed.